The moon phase disc shows you the position of the moon during its 29 1/2 day cycle.
New moon: moon is not visible
Waxing moon: moon is rising on the left side
Full moon: moon is visible in the center of the dial opening
Waning moon: moon is descending on the right side of the dial opening
Inside the back of the clock you can see the back bottom part of the moon disc. Turn this disc (if the shift mechanism is not engaged) to the correct position in the lunar phase.
The moon phase disc is shifted during part of each day. During this time you cannot turn the disc, pleae wait 4 - 6 hours and try again.